U.S. patent application number 11/135797 was filed with the patent office on 2006-11-30 for cerium-containing austenitic nickel-base alloy having enhanced intergranular attack and stress corrosion cracking resistances, and preparation method thereof.
This patent application is currently assigned to Korea Atomic Energy Research Institute. Invention is credited to Joung-Soo Kim, Oh Chul Kwon, Yong-sun Yi.
Application Number | 20060266449 11/135797 |
Document ID | / |
Family ID | 37461934 |
Filed Date | 2006-11-30 |
United States Patent
Application |
20060266449 |
Kind Code |
A1 |
Kwon; Oh Chul ; et
al. |
November 30, 2006 |
Cerium-containing austenitic nickel-base alloy having enhanced
intergranular attack and stress corrosion cracking resistances, and
preparation method thereof
Abstract
The present invention relates to a cerium-containing austenitic
nickel-base alloy having enhanced IGA(intergranular attack) and
SCC(stress corrosion cracking) resistances, and a preparation
method thereof. The cerium-containing alloy according to the
present invention is prepared by adding cerium to an austenitic
nickel-base alloy, which is used as a material of heat tubes of a
steam generator in a nuclear power plant. The cerium-containing
alloy has a refiner grain size than the conventional alloy without
cerium, a semi-continuous distribution of intergranular carbides,
and improved intergranular attack and stress corrosion cracking
resistances in the environment of high temperature and base.
Inventors: |
Kwon; Oh Chul; (Daejeon,
KR) ; Yi; Yong-sun; (Daejeon, KR) ; Kim;
Joung-Soo; (Daejeon, KR) |
Correspondence
Address: |
BACHMAN & LAPOINTE, P.C.
900 CHAPEL STREET
SUITE 1201
NEW HAVEN
CT
06510
US
|
Assignee: |
Korea Atomic Energy Research
Institute
Korea Hydro & Nuclear Power Co., Ltd
|
Family ID: |
37461934 |
Appl. No.: |
11/135797 |
Filed: |
May 24, 2005 |
Current U.S.
Class: |
148/677 ;
420/443 |
Current CPC
Class: |
C22C 19/03 20130101 |
Class at
Publication: |
148/677 ;
420/443 |
International
Class: |
C22C 19/05 20060101
C22C019/05; C22F 1/10 20060101 C22F001/10 |
Claims
1. A cerium-containing austenitic nickel-base alloy having the
cerium content of 0.02.about.0.09 wt %.
2. The cerium-containing austenitic nickel-base alloy of claim 1,
wherein the cerium content is 0.04 wt %.
3. The cerium-containing austenitic nickel-base alloy of claim 1,
wherein the alloy contains 58.about.80 wt % of nickel, 14.about.31
wt % of chromium, 6.about.11 wt % of iron, and 0.01.about.0.15 wt %
of carbon, as major components.
4. The cerium-containing austenitic nickel-base alloy of claim 1,
wherein the grain size of the alloy is 25.about.60 .mu.m.
5. The cerium-containing austenitic nickel-base alloy of claim 4,
wherein the grain size of the alloy is 35 .mu.m.
6. The cerium-containing austenitic nickel-base alloy of claim 1,
wherein the alloy has the mechanical properties of: 200.about.350
MPa of yield strength, 550.about.750 MPa of ultimate tensile
strength, and above 30% of elongation.
7. A preparation method of a cerium-containing austenitic
nickel-base alloy comprising the steps of: preparation of an ingot
by adding 0.02.about.0.09 wt % of cerium to an austenitic
nickel-base alloy composition; homogenizing heat treatment of the
ingot after surface grinding; preparation of a hot rolled material
by hot rolling the heat-treated ingot; removal of surface oxide
film by acid pickling the hot rolled material; preparation of a
cold rolled material by cold rolling the material after hot rolling
and removal of oxide film; solution annealing of the cold rolled
material; and thermal treatment of the solution annealing
material.
8. The preparation method of a cerium-containing austenitic
nickel-base alloy of claim 7, wherein the solution annealing is
performed at 900.about.1200.degree. C. for 5.about.60 min.
9. The preparation method of a cerium-containing austenitic
nickel-base alloy of claim 8, wherein the solution treatment is
performed at 1010.degree. C. for 10 min.
10. The preparation method of a cerium-containing austenitic
nickel-base alloy of claim 7, wherein the thermal treatment is
performed at 700.about.720.degree. C. for 10.about.20 hours.
11. The preparation method of a cerium-containing austenitic
nickel-base alloy of claim 10, wherein the thermal treatment is
performed at 704.degree. C. for 15 hours.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a cerium-containing
austenitic nickel-base alloy having enhanced IGA(intergranular
attack) and SCC(stress corrosion cracking) resistances, and a
preparation method thereof.
[0003] 2. Description of the Prior Art
[0004] The term of "austenite" means a solid solution of iron and
carbon, subsisting in a metal alloy of steel having a composition
of iron and carbon as a primary alloying material, above the
critical temperature of 1333.degree. F. (about 723.degree. C.). By
adding specific metals such as manganese, nickel or chromium to the
austenite, it is possible to obtain an austenitic structure
sustaining at room temperature and prepare austenitic steel.
[0005] Commercial atomic reactors currently running worldwide may
generally be classified into four types of: a pressurized water
reactor and a boiling water reactor developed in U.S., a high
temperature gas-cooled reactor developed in U.K., and a pressurized
heavy water reactor developed in Canada. In Korea, all the nuclear
power plants except Wolsung Nuclear Power Plant are utilizing the
pressurized water reactor. The pressurized water reactor (PWR) uses
slightly enriched uranium containing 2.about.5% of uranium-235 as a
nuclear fuel and water (light water) as a coolant and
moderator.
[0006] One of the accidents occurring frequently in nuclear power
plants utilizing the pressurized water reactor is a leakage from
heat tubes of a steam generator. The leakage from the heat tubes of
the steam generator is mainly caused from intergranular attack and
stress corrosion cracking in the steam generator. Such
intergranular attack and stress corrosion cracking occurring in the
heat tubes of the steam generator cause an accident of leakage from
primary cooling water and a stoppage of power plant operation, and
further cause repairing of damaged tubes or replacement of the
steam generator itself, which results in a lot of economical
loss.
[0007] The followings are reported as methods to improve the
intergranular attack and stress corrosion cracking resistances:
development of a system enabling prediction of defects occurring in
heat tubes, a thermal treatment method to improve a corrosion
characteristic by changing precipitation behavior of intergranular
carbides through mill annealing (MA) followed by thermal treatment
at about 700.degree. C. for 10.about.15 hours (Korea Patent No.
250810), a method to improve physical and mechanical properties of
material by thermomechanical processing (TMP) to modify
microstructure of crystal boundaries (U.S. Pat. Nos. 4,481,043
& 5,244,515), and a method to improve passive layer stability
by adding corrosion inhibitor, such as TiO.sub.2, TiB.sub.2,
CeB.sub.6 or LaB.sub.6, to the secondary side environment of the
steam generator (Korea Patent No.261665).
[0008] As a more substantial solution to improve resistance to
degradation threatening safety of the heat tubes, a study on
developing a new alloy has been started according to the
requirement of alloy having excellent resistance to the stress
corrosion cracking (U.S. Pat. Nos. 4,689,279 & 4,715,909). As a
result, Alloy 690 has been developed and utilized as an alternative
heat tube material for the steam generator since 1985. Alloy 690 is
a high-chromium nickel alloy having excellent resistance to a
variety of corrosive aqueous media and high-temperature
atmospheresis. High content of chromium in Alloy 690 provides high
resistance to oxidizing acids, especially to aqueous corrosion by
nitric acid and sulfidation at a high temperature. Besides the
above corrosion resistance, Alloy 690 has high strength, good
metallurgical stability and favorable fabrication characteristics.
Alloy 690 contains about 60% of nickel, about 30% of chromium, and
about 10% of iron as major components, and further contains
manganese (0.50% max.), sulfur (0.015% max.), silicon (0.50% max.),
copper (0.50% max.), carbon (0.05% max.) and cobalt (0.05% max.).
The stress corrosion cracking of Alloy 690 heat tubes has not been
reported yet. However degradation of the Alloy 690 heat tubes may
possibly happen due to the stress corrosion cracking, considering
that it takes quite a long time until problems related to the
corrosion can be reported after using a new material, and
considering the study result that Alloy 690 is possibly more
sensitive to lead-induced stress corrosion cracking (PbSCC).
Accordingly, a study for developing an alloy having an excellent
resistance to the intergranular attack and stress corrosion
cracking is still continued (Korea Patent No. 261665).
[0009] It has been reported that cerium improves high-temperature
oxidation characteristic when a small amount of cerium is added to
a Fe--Cr--Al alloy as an alloying element and a corrosion
characteristic in a basic solution when cerium is adsorbed to the
surface of a commercial steel material. Additionally, it has been
reported that micronization of grain in a surface molten layer is
obtained by laser clad coating of stainless steel type 316 with
powder containing CeO.sub.2 and, in the stainless steel type 316
containing a trace of cerium, cerium atoms can be distributed more
efficiently in grain boundaries having an irregular atomic
arrangement, because the atomic radius of cerium is very larger
than those of nickel, chromium, or iron, and thereby high-energy
sites required for precipitation of intergranular carbides may be
provided.
[0010] In a study for developing an alternative alloy, the
inventors completed the present invention by clarifying that an
alloy, having a more improved resistance to the intergranular
attack and stress corrosion cracking comparing to conventional
alloys, may be prepared by adding cerium to an austenitic
nickel-base alloy, and thereby used as a material of heat tubes for
a steam generator in a nuclear power plant.
SUMMARY OF THE INVENTION
[0011] The present invention provides an austenitic nickel-base
alloy containing 0.02.about.0.09 wt % of cerium.
[0012] The austenitic nickel-base alloy preferably contains
58.about.80 wt % of nickel, 14.about.31 wt % of chromium,
6.about.11 wt % of iron, and 0.01.about.0.15 wt % of carbon as main
components, and may further contain 0.05.about.0.15 wt % of
silicon, 0.001.about.0.004 wt % of manganese, 0.010.about.0.03 wt %
of phosphor, 0.005.about.0.01 wt % of cobalt, 0.003.about.0.007 wt
% of titanium, 0.001.about.0.005 wt % of copper, 0.005.about.0.05
wt % of aluminum, and 0.001.about.0.1 wt % of boron.
[0013] Additionally, the austenitic nickel-base alloy preferably
contains 0.0.about.0.09 wt % of cerium, and more preferably 0.04 wt
% of cerium.
[0014] Additionally, grain size of the austenitic nickel-base alloy
containing cerium is preferably 25.about.60 .mu.m, and more
preferably 35 .mu.m.
[0015] Additionally, intergranular carbide coverage of the
austenitic nickel-base alloy containing cerium is preferably
60.about.70%.
[0016] Additionally, the austenitic nickel-base alloy containing
cerium has the mechanical properties of: 200.about.350 MPa of yield
strength, 550.about.750 MPa of ultimate tensile strength, and above
30% of elongation.
[0017] Hereinafter, a role and a preferable content of each
alloying element used in the preparation of the alloy according to
the present invention will be described in detail.
[0018] Cerium is harder than tin and softer than zinc. It is well
known that mechanical properties and ductility of alloy are
improved by adding cerium into the alloy. Cerium oxide is used as
an abrasive material or catalyst. In the present invention, cerium
is used as an alloying element to improve resistance of alloy to
intergranular attack and stress corrosion cracking. It is reported
that intragranular carbides precipitate more when cerium is added
less than 0.01 wt % and the intergranular carbides contributing to
the stress corrosion cracking resistance precipitate when cerium is
added more than 0.04 wt % [Y. Watanabe, Scripta mater., 42,
307-312, 2000]. The cerium-containing alloy preferably contains
0.02.about.0.09 wt % of cerium, and more preferably 0.04 wt %.
[0019] Nickel and chromium reinforce oxide film, and thereby
improve corrosion resistance of alloy. Especially, nickel is an
austenite stabilizing element, which accelerates deformation from
BCC (body-centered cubic) to FCC (face-centered cubic). Nickel
delays formation of an intermetallic compound in
austenite-stainless at the room temperature, and thereby stabilizes
the structure of the austenite. Nickel and chromium respectively
form precipitates such as Ti.sub.2Ni and Ti.sub.2Cr and reduce
over-potential of hydrogen, and thereby reinforce titanium oxide
film. Additionally, such precipitates in the oxide film reduce a
current density required for maintaining a passive state.
[0020] It is reported that nickel and chromium give a great
influence in reinforcing and stabilizing the passive state of
titanium, if added together with an element of platinum family.
Additionally, addition of nickel and chromium reduces the addition
of an expensive platinum family element, and thereby enables
manufacturing an economical alloy. The alloy containing cerium in
accordance with the present invention preferably contains
58.about.80 wt % of nickel and 14.about.31 wt % of chromium.
[0021] Iron is added as a major element like chromium to improve
the corrosion resistance of the alloy, and the cerium-containing
alloy in accordance with the present invention preferably contains
6.about.11 wt % of iron.
[0022] Carbon and silicon reduce absorption of hydrogen and delay
transition time of corrosion, and the cerium-containing alloy in
accordance with the present invention preferably contains
0.01.about.0.20 wt % (max.) of carbon and 0.05.about.0.15 wt % of
silicon, as impurity elements related to the corrosion
resistance.
[0023] Manganese enables an improvement of alloy strength without
decreasing the corrosion resistance, and further reduces or
inhibits an adverse reaction of iron. Additionally, manganese is
effective for micronization of grain size and reduction of electric
conductivity, and generally used as an important metal component of
alloy. The cerium-containing alloy in accordance with the present
invention preferably contains 0.001.about.0.004 wt % of
manganese.
[0024] Phosphor is a very active element forming a compound with
copper and behaves as a very strong oxidant for oxygen. When
phosphor melts, a wide, thin and strong film is formed on a fusion
surface and an alloy containing both phosphor and copper shows a
high tensile strength. The cerium-containing alloy in accordance
with the present invention preferably contains 0.010.about.0.03 wt
% of phosphor.
[0025] Cobalt is one of the important elements of alloy and
improves oxidation and corrosion resistances. The cerium-containing
alloy in accordance with the present invention preferably contains
0.005.about.0.01 wt % of cobalt.
[0026] Titanium is a metal with a silver white color and has a
light weight next to magnesium and aluminum. Titanium has a high
ductility and excellent mechanical properties. Specific strength of
titanium is 2 times higher than that of iron and 6 times higher
than that of aluminum. Titanium has an excellent thermal
resistance, and titanium has a high yield point up to 500.degree.
C. Titanium also has a very excellent resistance to acid or
seawater, and is thereby used as an alloying element. The
cerium-containing alloy in accordance with the present invention
preferably contains 0.003.about.0.007 wt % of titanium.
[0027] Copper is added as a major component element like iron or
chromium to improve the corrosion resistance of alloy, and has an
excellent effect especially when a very small amount is added.
Accordingly, the cerium-containing alloy in accordance with the
present invention preferably contains 0.001.about.0.005 wt % of
copper.
[0028] Aluminum has a chemical characteristic of forming a stable
oxide film in the air through corrosion caused by inorganic acid or
salts, and is thereby used in manufacturing an alloy together with
various alloying elements.
[0029] Boron has the radius less than 1 .ANG. and improves the
hardness of alloy by penetrating into crystal grains of alloy. The
cerium-containing alloy in accordance with the present invention
preferably contains 0.001.about.0.1 wt % of boron.
[0030] Grain size of the cerium-containing austenitic nickel-base
alloy prepared by the above composition in accordance with the
present invention is preferably 25.about.60 .mu.m, and more
preferably 35 .mu.m. The grain size is indicated with an average
diameter of the grain or grain fineness number, which is a very
important factor influencing various mechanical properties of metal
material. The grain sizes may be classified into two. One is a
ferrite grain size, which indicates a grain size in pure iron or
low-carbon steel and gives a great influence to hardness and deep
drawing property of the alloy. The other is an austenitic grain
size existing in a high temperature before transforming to a grain
size in high-carbon steel or low alloy steel.
[0031] Additionally, the cerium-containing austenitic nickel-base
alloy in accordance with the present invention preferably has the
intergranular carbide coverage of 60.about.70%. The intergranular
carbide coverage is calculated by dividing the length of grain
boundary into the total length of the intergranular carbide, and is
used as a factor of measuring a primary water intergranular stress
corrosion cracking (PWSSC) resistance of Alloy 600. Therefore, the
intergranular carbide coverage has been applied as a factor of
measuring a SCC resistance in a base environment, in the present
invention.
[0032] Additionally, the cerium-containing austenitic nickel-base
alloy in accordance with the present invention preferably has
mechanical properties of: 200.about.350 MPa of yield strength,
550.about.750 MPa of maximum tensile strength, and above 30% of
elongation. In a tensile test of a material, when a stress is
increased above the limit of forgebility, deformation of the
material is rapidly increased resulting in plastic deformation.
Stress and strength of the material at this point are called
respectively yield stress and yield strength. The maximum tensile
strength is also called ultimate tensile strength, and means the
maximum stress that the material can sustain. In a graph showing
correlation between tensile load and deformation by stretching a
material, the deformation of the material initially increases in a
linear form as the tensile load increases until reaching a yield
point, and plastic deformation starts after passing the yield
point. An increase rate of the deformation then slows down and,
after passing a maximum point, the tensile load is reduced due to
the decrease of material cross-section by stretching, and the
material finally breaks down. A stress at the point where the
tensile load becomes a maximum is the tensile strength of the
material, which is distinguished from fracture strength as a stress
when the material breaks down. Additionally, the elongation means
an extent that a material is stretched in a tensile test.
[0033] The present invention also provides a preparation method of
a cerium-containing austenitic nickel-base alloy comprising the
steps of:
[0034] preparation of an ingot by adding cerium to an austenitic
nickel-base alloy composition;
[0035] homogenizing heat treatment of the ingot after a surface
grinding work;
[0036] preparation of a hot rolled material by hot rolling (hot
working);
[0037] removal of surface oxide film by acid pickling the hot
rolled material;
[0038] preparation of a cold rolled material by cold rolling (cold
working);
[0039] solution annealing; and
[0040] thermal treatment.
[0041] Hereinafter, the preparation method of a cerium-containing
austenitic nickel-base alloy will be described in more detail.
[0042] Firstly, an ingot is prepared after mixing and melting
compositions of nickel, chromium, iron, carbon, and cerium
according to a specific ratio. Vacuum induction melting provides a
uniform structure of the ingot by a self-mixing action of inductive
heating. The vacuum induction melting has the following advantages.
Degasification is increased as an effect of vacuum heating, and may
be further increased by maintaining the vacuum state for a specific
period after melting. The degasification is performed without
adding antioxidant, and excellent cleanliness is thereby
maintained.
[0043] Secondly, the ingot is processed by a surface grinding work
and homogenizing heat treatment to remove impurities on the surface
of the ingot. After the surface grinding work, homogenizing heat
treatment is performed for a long period at the temperature range
of austenite in order to remove segregation by diffusion of atoms
and to homogenize the components. The homogenizing heat treatment
is generally performed for 1 hour per inch (2.54 cm) of thickness,
and performed preferably at 1100.about.1300.degree. C. for
60.about.180 min., and more preferably 1250.degree. C. for 135 min.
to get a satisfactory result.
[0044] Thirdly, after the homogenizing heat treatment, the ingot is
hot rolled above the recrystallization temperature of the metal
material.
[0045] Fourthly, the ingot is acid pickled to remove oxide film on
the surface of the hot rolled material. In the acid pickling
process, a solution mixed with H.sub.2O, HNO.sub.3, HCl and
H.sub.2O.sub.2 is preferably used, and a solution mixed with the
volume ratio of H.sub.2O:HNO.sub.3:HCl:H.sub.2O.sub.2=3:2:2:1 is
used more preferably.
[0046] Fifthly, a cold rolled material is prepared by cold rolling
the hot rolled material. The purpose of the cold rolling is to
increase the strength of the material, which is different from the
purpose of the hot rolling to form the material. The cold rolling
increases hardness and strength of material, accompanying a
decrease of ductility.
[0047] Sixthly, the cold rolled material is solution annealing in a
proper condition so that the grain size of the material becomes
uniform. The alloy used for the heat tubes of the steam generator
requires the average grain size above 32 .mu.m approximately.
Therefore, it is important to set a solution annealing condition so
that carbides do not exist in intragranule and intergranule by
removing cold rolling structure and solubilizing carbides
completely into the matrix. Accordingly, the cerium-containing
alloy in accordance with the present invention is preferably
solution annealing at 900.about.1200.degree. C. for 5.about.60 min.
to obtain the uniform grain size above 32 .mu.m, and more
preferably at 1010.degree. C. for 10 min.
[0048] Lastly, thermal treatment (TT) is applied to the alloy in
order to increase the SCC resistance. It is reported that the
thermal treatment generally performed to increase the SCC
resistance of Alloy 600, which is used as heat tubes of the steam
generator improves the SCC resistance of material due to
precipitating optimum intergranular carbides and distributing the
intergranualr carbides semi-continuously [J. R. Crum, Corrosion,
38, 40-45, 1982], and is most effective when final thermal
treatment is performed at 704.degree. C. for 15 hours. Accordingly,
the final thermal treatment of the cerium-containing austenitic
nickel-base alloy in accordance with the present invention
preferably is applied at 700.about.720.degree. C. for 10.about.20
hours, and more preferably at 704.degree. C. for 15 hours.
[0049] The cerium-containing austenitic nickel-base alloy in
accordance with the present invention has more carbide precipitates
distributed semi-continuously than the comparative example without
cerium, and may thereby have more improved IGA and SCC resistances.
Additionally, the SCC resistance of heat tubes in an operating
condition of the steam generator of the nuclear power plant may be
improved. Accordingly, the cerium-containing austenitic nickel-base
alloy in accordance with the present invention may be used
effectively as a material of heat tubes in the steam generator of
the nuclear power plant. By using this material for the heat tubes
of the steam generator, breakages of the heat tube due to the
intergranular attack and stress corrosion cracking may be
decreased, and thereby the safety and economical efficiency may
also be improved.
BRIEF DESCRIPTION OF THE DRAWINGS
[0050] FIG. 1 is a graph showing a trend of grain sizes of a
comparative example and an alloy containing cerium (Ce) according
to solution annealing condition.
[0051] FIG. 2 is a micrograph showing microstructures of the
comparative example and alloy containing cerium after the solution
annealing in the present invention.
[0052] FIG. 3 is a graph showing a trend of intergranular carbide
coverage of the comparative example and alloy containing cerium
after the thermal treatment at 704.degree. C. for 15 hours.
[0053] FIG. 4 is a micrograph of SEM showing the intergranular
carbides of the comparative example and alloy containing cerium
after the thermal treatment at 704.degree. C. for 15 hours.
[0054] FIG. 5 is a micrograph of TEM showing selective area
diffraction pattern of the intergranular carbides of comparative
example and alloy containing cerium after the thermal treatment at
704.degree. C. for 15 hours.
[0055] FIG. 6 is a graph showing a trend of chromium concentration
in the intergranule of the comparative example and alloy containing
cerium after the thermal treatment at 704.degree. C. for 15
hours.
[0056] FIG. 7 is a graph showing stress-strain curves of the
comparative example and alloy containing cerium after the thermal
treatment at 704.degree. C. for 15 hours.
[0057] FIG. 8 is a graph showing a test result of stress corrosion
cracking of the comparative example and alloy containing cerium in
the environment of 40% NaOH solution.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0058] Hereinafter, a preferred example embodiment and a
comparative example will be described in detail to help the
understanding of the present invention. However, the following
example embodiment and comparative example are provided for an
easier understanding of the present invention, and this invention
should not be construed as limited to the example embodiment set
forth herein or in the accompanying drawings.
EXAMPLE
Preparation of Cerium-Containing Austenitic Nickel-Base Alloy
[0059] A 30 kg ingot is prepared by melting and tapping a mixture
without cerium and a mixture with a trace of cerium in vacuum
induction melting, using nickel of 99.90% purity, chromium of
99.48% purity, iron of 99.93% purity, carbon and high purity
cerium. Hot rolled plates with the thickness of 7 mm are prepared
by cutting the ingot into half pieces, surface grinding, keeping at
1250.degree. C. for 135 min., and hot rolling. Acid pickling of the
hot rolled plates are applied to remove surface oxide film, using a
solution mixed with the volume ratio of
H.sub.2O:HNO.sub.3:HCl:H.sub.2O.sub.2=3:2:2:1, and cold rolled
plates with the final thickness of 1.5 mm are prepared by cold
rolling. The compositions of the prepared alloy are shown in Table
1.
[0060] As a subsequent process, the prepared alloy is solution
annealing so that the grain size of the alloy becomes about 35
.mu.m. In order to analyze an effect of the solution annealing
temperature and time influencing the grain size of the alloy,
microstructure of the alloy is observed after the solution
annealing at 900.about.1100.degree. C. for 30 min. Based on the
above result of the experiment, the comparative example and
cerium-containing alloy are solution annealing respectively at
975.degree. C. for 20 min. and at 1010.degree. C. for 10 min.
[0061] As the last step, in order to investigate an influence of
thermal treatment time to the precipitation of the carbides, the
thermal treatment of the solution annealing alloy is applied at
704.degree. C. for 1.about.96 hours, and distribution of the
carbides is observed, and finally the thermal treatment of the
solution annealing alloy is applied at 704.degree. C. for 15 hours,
based on the above result of the experiment.
Comparative Example
Preparation of Austenitic Nickel-Base Alloy without Cerium
[0062] Another alloy is prepared with the same method as the above
example embodiment except the step of adding cerium. The
compositions of the prepared alloy are shown in Table 1.
TABLE-US-00001 TABLE 1 Composition of austenitic nickel-base alloy
with and without cerium (wt %) Comparative Embodiment Example
Example C 0.03 0.03 Si 0.07 0.12 Mn 0.003 0.002 P 0.024 0.018 Cr
16.12 16.15 Ni 74.42 74.36 Fe 9.06 9.04 Co 0.007 0.007 Ti 0.005
0.005 Cu 0.003 0.002 Al -- 0.02 B 0.006 0.03 Ce 0.003 0.04
[0063] The example and comparative example are prepared as shown in
Table 1. It is reported that precipitation of intragranular
carbides is increased when the cerium content is less than 0.01 wt
% and intergranular carbides contributing to the SCC resistance
precipitates when the cerium content is more than 0.04 wt % [Y.
Watanabe, Scripta mater., 42, 307-312, 2000].
[0064] Accordingly, in the present invention, 0.04 wt % of cerium
is added, which may contribute to the precipitation of
intergranular carbides.
Experiment 1: Observation of Microstructure
[0065] In order to observe the microstructure of the solution
annealing alloy, the alloy of the example embodiment is
electrically etched in a nital solution at 4.5V DC for 30 sec., and
microstructure of the alloy is observed by an optical microscope.
The average grain size is measured by the linear intercept method
from the optical micrograph.
[0066] The results of the alloys solution annealing at
900.about.1100.degree. C. for 30 min. according to the example
embodiment are shown in FIG. 1. FIG. 2 shows micrographs of the
comparative example solution annealing at 975.degree. C. for 20
min. and the cerium-containing alloy solution annealing at
1010.degree. C. for 10 min.
[0067] As shown in Table 1, according to the result of measuring
grain size, the grain size of the embodiment example is smaller
than that of the comparative example. This may be interpreted as
the addition of trace cerium induces refinement of the grain size.
Additionally, it is clarified that the comparative example and
embodiment example should be solution annealing respectively at
975.degree. C. for 20 min. and at 1010.degree. C. for 10 min., in
order to obtain the grain size of about 35 .mu.m, both for the
comparative example and embodiment example.
[0068] As shown in FIG. 2, it is clarified that carbides of the
comparative and embodiment examples are solubilized into a matrix
respectively at 975.degree. C. for 20 min. and 1010.degree. C. for
10 min., and thereby microstructure having a uniform grain size may
be obtained.
Experiment 2: Analysis of Intergranular Carbides
[0069] In order to evaluate the influence of cerium after the final
thermal treatment to the distribution of the intergranular
carbides, intragranular and intergranular carbides of both alloys
after the thermal treatment are investigated and analyzed by a
scanning electron microscope (SEM) and a transmission electron
microscope (TEM).
2-1: SEM Analysis
[0070] The embodiment example and the comparative example etched by
2% bromine solution (2% bromine+98% methanol) are investigated by
the SEM and taken micrographs. A trend of intergranular carbides
precipitation is investigated by measuring the lengths of the
intergranule and the intergranular carbide and quantifying the
coverage of the intergranular carbide. The coverage of the
intergranular carbide is calculated by dividing the total length of
intergranule into the total length of the intergranular carbide.
The result is shown in FIG. 3.
[0071] As shown in FIG. 3, according to the result of comparing the
coverage of the embodiment example with that of comparative
example, the coverage of the intergranular carbide of embodiment
example is increased by about 10% comparing to that of the
comparative example. Therefore, it is clarified that the embodiment
example containing a trace of cerium has higher coverage of the
intergranular carbide than that of the comparative example. This
indicates that the IGA and SCC resistances may be improved, because
the embodiment example has more intergranular carbide distributed
semi-continuously than the comparative example.
[0072] Additionally, as shown by SEM micrographs in FIG. 4, it is
clarified that the intergranualr carbides both for embodiment
example and comparative example has the highest coverage and is
distributed semi-continuously by the thermal treatment at
704.degree. C. for 15 hours.
2-2: TEM Analysis
[0073] In order to analyze the composition and crystal structure of
the intragranular and intergranular carbides, only carbides are
extracted by a carbon replica method [Y. S. Lim, A study on the
microstructure and corrosion characteristics of laser surface
melted Ni-base Alloy 600, Ph.D., Thesis, KAIST, 2000] and analyzed
with EDS (Energy Dispersive X-ray Spectroscopy, Oxford Link
ISIS-5947) of TEM (Transmission Electron Microscope, JEOL
JSM-2000FX).
[0074] According to the analysis result, the carbide of the
comparative example contains 98.4 wt % of chromium and 1.6 wt % of
iron, and the carbide of embodiment example contains 96.9 wt % of
chromium, 2.0 wt % of iron, and 1.2 wt % of nickel. Referring to
FIG. 5, both alloys shows selected area diffraction patterns having
continuous streaks parallel to the diffraction planes, which
typically appears in the carbide of Cr.sub.7C.sub.3. From the
compositions and selected area diffraction patterns, both alloys
are identified as the carbides have a pseudo hexagonal structure of
Cr.sub.7C.sub.3. As shown in FIG. 6, according to the result of
investigating the change of chromium concentration in the
intragranule, both alloys doesn't show any significant
difference.
Experiment 3: Analysis of Mechanical Characteristics
[0075] Tensile tests are performed to analyze mechanical
characteristics of the alloy prepared by the present invention.
[0076] A tensile testing machine of Model Instron 8872 is utilized
at room temperature. Cross head speed is set to 2 mm/min, and a
specimen is provided with 25 mm of gauge length, 1.5 mm of gauge
thickness, and 4 mm of width, using electric discharge machining.
The result of the analysis is shown in Table 2 and FIG. 7.
[0077] As shown in Table 2 and FIG. 7, according to the result of
the tensile tests for the comparative example and the embodiment
example, both alloys doesn't show any significant difference in
0.2% offset yield strength (YS), ultimate tensile strength (UTS),
and elongation. Accordingly, it is clarified that the trace cerium
does not give any significant influence to the mechanical
properties of the austenitic nickel-base alloy. TABLE-US-00002
TABLE 2 0.2% offset Ultimate yield tensile strength strength
Elongation (Mpa) (Mpa) (%) Comparative 212 (.+-.1) 677 (.+-.2) 40.4
(.+-.1.2) example Embodiment 212 (.+-.2) 694 (.+-.1) 40.6 (.+-.0.2)
example
Experiment 4: Measurement of SCC Resistance
[0078] In order to compare and evaluate the SCC resistances of the
comparative example and the embodiment example, SCC tests are
carried out using a U-bend specimen in a simulated model of crevice
in the secondary side of the steam generator of the nuclear power
plant.
[0079] The U-bend specimens of alloy are prepared both for the
comparative example and the embodiment example after thermal
treatment at 704.degree. C. for 15 hours, according to ASTM
(American Society for Testing and Materials) G30-79. An environment
of the most frequently causing the stress corrosion cracking in the
secondary side of heat tube of the steam generator is a highly
basic condition at the operating temperature of the steam
generator. Therefore, the test is carried out at 315.degree. C.
with 40% NaOH solution in an autoclave with 2 liters capacity.
Additionally, in order to accelerate the stress corrosion cracking,
the test is carried out by applying a voltage 200 mV higher than
the corrosion potential of the material. After completing the test,
the U-bend specimen is divided into 3 pieces equally, and the
shape, depth, and number of cracks were investigated. The result of
measurement is shown in FIG. 8.
[0080] As shown in FIG. 8, in the case that SCC test time is 30
hours, the average depth of maximum crack of the comparative
example is 184 .mu.m, however the embodiment example doesn't show
any crack. Additionally, in the case that SCC test time is 60
hours, the average depth of maximum crack in the comparative
example is 1,279 .mu.m, whereas that in the embodiment example is
946 .mu.m. From the result of the SCC test, it is verified that the
embodiment example delays starting time of cracking and has an
excellent SCC resistance comparing to the comparative example.
[0081] As described above, it is verified that the grain is
refined, more intergranular carbides is distributed
semi-continuously, and SCC resistance is improved, by adding cerium
into an austenitic nickel-base alloy.
[0082] Accordingly, the new alloy having improved IGA and SCC
resistances may be applied as a material for heat tubes of a steam
generator in a nuclear power plant, and will contribute to
improvement of the safety and economical efficiency in a nuclear
power plant by reducing breakages of heat tubes caused from the
intergranular attack and stress corrosion cracking.
* * * * *